The present invention relates to a heat insulation material on a ceramic base as well as to a method of coating a substrate with such a heat insulation material.
Ceramic heat insulation materials are in particular suitable for high-temperature use and therefore used, for example, in gas turbine components, such as combustion chamber elements and turbine vanes, or in engine cylinders. It is the function of the heat insulation materials to protect the substrates coated with them from temperatures which are too high. This is, for example, necessary with inner walls of engine cylinders which are exposed to ever higher temperatures in operation due to the endeavor to increase efficiency and thereby to lower the specific fuel consumption of the engine, the temperatures often reaching or surpassing the limits of use of the base materials despite a direct cooling and a specific construction of the components.
As a rule, the heat insulation materials include a functional layer of ceramic material, for example of Y2O3 doped ZrO2. Due to the high porosity and the high diffusion coefficient for oxygen, the functional layer, however, frequently does not protect the substrate material or at least does not protect it sufficiently against oxidation or hot gas corrosion. In addition the adhesion of the Y2O3 doped ZrO2 layer on the substrate, for example the base material of a turbine vane, is not sufficient. For this reason, the known heat insulation materials usually include an adhesion promotion layer which is arranged beneath the functional layer and which should inter alia ensure the required adhesion of the functional layer on the substrate.
However, a failure of the layer systems occurs in the known layer systems due to different thermal coefficients of expansion, insufficient phase stability, the growth and the behavior of the thermally grown oxide layer on the adhesion layer, an insufficient oxidation resistance and hot gas corrosion resistance and further influences, in particular under highly fluctuating conditions of use, i.e. under conditions of use in which high and low temperatures change periodically. There are different types of failure in dependence on the type of strain. Two important breakdown mechanisms are failure in the ceramic material at high temperatures and the peeling of parts of the heat insulation layer at rather low strain temperatures. Sintering behavior and phase stability play an important role for the failure in the ceramic material. The failure mechanism at the rather low temperatures is closely associated with the growth of the oxide layer (TGO) on the adhesion promotion layer.
A heat insulation material is, for example, known from DE 198 01 424 A1 which can in particular be used at temperatures of more than 1,000° C. and which consists of a top layer substantially consisting of BaZrO3 and/or La2Zr2O7 and/or SrZrO3 and an intermediate layer or adhesion promotion layer arranged thereunder made of an MeCrAlY alloy, where Me=Ni or Co.
A thermal insulation layer is disclosed in DE 101 58 639 A1 on the basis of La2Zr2O7 in which 10 to 90% of the lanthanum is replaced by neodymium, dysprosium, samarium or europium, the heat insulating layer being applied by plasma spraying onto an adhesion promotion layer of MeCrAlY applied to a substrate.
However, the two aforesaid heat insulation materials are also in need of improvement with respect to their heat insulation, in particular under conditions of use which change a lot thermally or under cyclic temperature strain.